It is known that numerous airports are equipped with a ground-based radionavigation system emitting signals allowing instrument precision landing of the ILS (“Instrument Landing System”) type of an aircraft, in particular of a civil transport plane, equipped with appropriate receivers.
Such a radionavigation system comprises ground stations that are situated at the strip verge and at least one specialized radioreceiver mounted aboard the aircraft, which provides horizontal and vertical guidance before and during landing by presenting the pilot with the lateral deviation with respect to an approach axis and the vertical deviation with respect to a descent plane. Said radionavigation system generally comprises, as ground stations:                a directional UHF radiotransmitter, placed downstream of the strip threshold and on one side or the other of the axis of the strip, and intended to ensure vertical guidance along an approach axis according to an ideal plane of descent during an “ILS” approach, by virtue of the emission of an appropriate vertical guidance beam (“Glide” beam). More precisely, this radiotransmitter emits two signals with different modulation which are aligned one above the other and which overlap on the approach axis where the two signals are received with equal strength; and        a directional VHF radiotransmitter, generally placed on the strip axis at the opposite end from the approach threshold, and intended to ensure azimuthal guidance along the approach axis according to an ideal lateral alignment profile during an “ILS” approach, by virtue of the emission of an appropriate lateral alignment beam (“Loc” beam). More precisely, this radiotransmitter emits two signals with different modulation which overlap on the approach axis where the two signals are received with equal strength.        
Such a radionavigation system affords considerable and effective assistance with landing (through lateral guidance and vertical guidance), in particular through poor visibility (fog, etc.) or in the absence of visibility.
However, it may happen that such a radionavigation system is incomplete or at least partially faulty so that the pilot then no longer has access to all the information allowing completely assisted landing.
The present invention applies to the case where no “glide” beam (vertical guidance) is available, only a “loc” beam (lateral guidance) being detectable from the aircraft.
Such a case may occur in particular in the following situations:                the radionavigation system comprises only means for emitting a “loc” beam;        the means of said radionavigation system which are intended for emitting a “glide” beam are faulty;        the radionavigation system emits a “glide” beam forward of the strip and “loc” beams forward, as well as rearward of the strip, and the landing is carried out from the rear.        
When such a case occurs, the pilot must manage the vertical guidance without assistance. Such management requires a considerable workload and close attention on the part of the pilot, and this may sometimes increase the risk of errors and hence entail lesser flight safety than during an “ILS” type precision approach, for which, as indicated above, the pilot benefits in particular from assistance in respect of vertical guidance.